For some time colour television display tubes have been used in which the three spatially separated electron guns are situated on one line. Such a display tube is known as an in-line colour display tube. In the in-line colour display tube it is usual to use a deflection yoke with deflection coils which give such an inhomogeneous field distribution that the beams of the electron guns upon deflection coincide all over the screen. For that purpose the line deflection magnetic field on the cup side of the deflection yoke (the side which corresponds to the end of the core having the larger diameter) must be pin-cushion-shaped and the field deflection magnetic field must be barrel-shaped on the neck side (the side which corresponds to the end of the core having the smaller diameter) and conversely.
The extent of pin-cushion-shape and barrel-shape is such that convergence errors of the electron beams emitted by the electron guns are corrected, so that pictures with satisfactory convergence properties are produced on the screen of the display tube. Display tube/deflection yoke combinations of this type are termed self-converging.
When the convergence is ensured in this manner, it often appears to be necessary to correct a pin-cushion distortion (east-west raster distortion) occurring on the vertical sides of the display screen.